A thermistor is a material whose resistance varies with temperature. "PTC" thermistors ("positive temperature coefficient") increase their resistivities within a particular temperature range in the vicinity of their ferroelectric Curie temperature. To be practical, PTC thermistors should possess a room-temperature resistivity of no greater than about 5,000 ohm-centimeters and, additionally, should have a resisitivity increase at the ferroelectric transition temperature of greater than about three orders of magnitude; see, e.g., "Advances in Ceramics," Volume 1, "Grain Boundary Phenomena in Electronic Ceramics" (American Ceramic Society, Columbus Ohio, 1981), pages 138-154, the disclosure of which is hereby incorporated by reference into this specification. "NTC" ("negative temperature coefficient") thermistors decrease their resistivities with temperature; see, e.g., E. D. Macklen, "Thermistors," (Electrochemical Publications Ltd., 1979), the disclosure of which is also incorporated by reference into this specification.
One common problem with PTC thermistors is that their properties degrade in the presence of either vacuum or reducing/inert atmospheres. Thus, as is disclosed in an article by B. M. Kulwicki entitled "Instabilities in PTC Resistors" (Proc. of the 6th IEEE International Symposium on Applications of Ferroelectricts, Lehigh University, Bethlehem, Penna., June, 1986, at pages 656-664), the disclosure of which is hereby incorporated herein by reference, the magnitude of the increase in resistivity with increasing temperature decreases substantially when the PTC thermistors are annealed in the presence of vacuum or reducing/inert atmospheres. This instability is also disclosed in the aforementioned "Advances in Ceramics", "Grain Boundary Phenomena. . ." book cited above; see, for example, pages 141 and 152. Thus, as is disclosed on page 152 of said book, a PTC thermistor whose resistivity at 120 degrees centigrade is normally about 100 ohm-centimeters and whose resistivity normally increases to 100,000 ohm-centimeters at 140 degrees centigrade will, when subjected to vacuum at 315 degrees centigrade, only increase its resistivity to 10,000 ohm-centimeters at 140 degrees centigrade. Thus, its property degrades by a factor of 10. If the annealing temperature is higher than 315 degrees centigrade, its property will usually degrade by substantially more than a factor of 10.
In addition to producing PTC thermistors which are substantially unstable, the prior art processes suffer another disadvantage--the PTC thermistors produced by them do not have properties which are readily and consistently reproducible on a large scale basis.
Barium titanate is a material commonly used to produce PTC thermistors. The prior art teaches that, in order to obtain the PTC effect with barium titanate material, it is essential to use chemically doped material. Thus, in a paper by J. Daniels et al. entitled "The PTC effect of barium titanate," (Philips Technical Review, Volume 38, 1978/79, No. 3), it is disclosed at page 81 that: "The second question was; why is the PTC effect not found in undoped BaTiO.sub.3 that has been made N-type by a reducing treatment? The answer to this is: under these conditions the material has become N-type due to the formation of oxygen vacancies, and there are no or hardly any barium vacancies present, whose behaviour is a necessary condition for the formation of grain-boundary layers." Thus, e.g., at page 142 of the aforementioned Kulwicki article (appearing in the "Advances in Ceramics book), it is disclosed that "Ceramic BaTiO.sub.3-x does not exhibit a PTC anomaly."
Another prior art reference indicating that undoped barium titanate will not exhibit the PTC effect is an article by I. Ueada et al., Journal of the Physical Society of Japan, Vol. 20, No. 4, April, 1965, at pages 546-552. This paper discusses an experiment in which undoped polycrystalline barium titanate was subjected to a temperature of 1,000 degrees centigrade under pure hydrogen for 5 hours. With regard to this sample, the authors stated (at page 552) that "No anomaly is observed in reduced polycrystalline BaTiO.sub.3.
The prior art also discloses that, if one has a barium titanate material which exhibits the PTC anamoly (which, must be a chemically doped barium titanate), the increase of the resistivity near the Curie temperature of the material can be increased by halogenating the material. See, e.g., a paper by G. Jonker, "Halogen Treatment of Barium Titanate Semiconductors," Mat. Res. Bull. Vol. 2, pp. 401-407, Pergammon Press Inc.
One problem involving the chemical doping of barium titanate is that, because of the small amounts ( on the order of about 0.2 atom percent) of dopant required, it is very difficult to obtain a homogeneous doped product; unless the dopant is uniformly substituted throughout the barium titanate crystal lattice (and not only on its surface), inhomogeneity will result, and the desired n-type semiconduction (which is essential for the PTC effect) will not be achieved. Another problem involving such chemical doping is that the properties obtainable with the process are not readily and consistently reproducible on a large scale basis.
It is an object of this invention to provide a process for preparing a PTC thermistor with substantially improved stability properties.
It is another object of this invention to provide a process for the production of said PTC thermistor which does not require the use of chemically doped barium titanate.
It is yet another object of this invention to provide a process for the production of said thermistors which provides reproducible results on a large scale basis.
It is yet another object of this invention to provide a process which can be used not only to prepare said PTC thermistors with improved properties but also varistors and capacitors.